Abstract

The reptilian group Lepidosauria diversified through the Mesozoic, survived the end-Cretaceous extinction relatively unscathed, and has more than 7,000 living species. Although originally constituted as a “waste-bin” for non-archosaurian diapsids, modern definitions limit Lepidosauria to its two constituent groups, Rhynchocephalia and Squamata, and their most recent common ancestor. To date, the earliest known lepidosaurs are from the Late Triassic (Carnian) of Europe and India, but their derived morphology provides indirect evidence of a longer, unrecorded, history. Rhynchocephalians and squamates probably diverged in the Early-Middle Triassic, and new material from the Early Triassic of Poland sheds some light on their common ancestor. The roots of Lepidosauria may extend into the Palaeozoic, but there are critical gaps in the fossil record.

Today Rhynchocephalia is limited to the New Zealand Tuatara, Sphenodon, but rhynchocephalians may have been the first lepidosaurs to undergo a major radiation. They had a global distribution in the Late Triassic, and diversified through the Jurassic and Early Cretaceous, expanding into new terrestrial, aquatic, and trophic niches. However, the group apparently went extinct in Asia during the first half of the Jurassic and then disappeared from Euramerica records in the mid-Cretaceous. This contrasts with Gondwanan records, as evidenced by recent discoveries in the Late Cretaceous of South America.

In many ways, our knowledge of Mesozoic squamate history is the reverse of that for Rhynchocephalia. With the exception of the Indian Tikiguana, there is no record of Triassic squamates, and only in the Early Cretaceous does the record improve significantly. Even so, most data comes from Laurasia. Very little is known of contemporaneous assemblages in Gondwana, making it difficult to reconstruct centres of origin for major squamate clades, although the relative abundance of snakes in Late Cretaceous of Gondwana is suggestive. For many years, our understanding of squamate evolution was obfuscated by an over reliance on characters of the temporal region, like the absence of a lower temporal bar. As a result, the fully diapsid skull of Sphenodon was considered primitive, and several small diapsids were mistakenly attributed either to Squamata or to its immediate stem. In fact, the fossil evidence demonstrates that the ancestral lepidosauromorph lacked a lower temporal bar. Rhynchocephalians and squamates each modified this ancestral condition in different ways.

Keywords

Late Cretaceous Early Cretaceous Middle Jurassic Middle Triassic Early Jurassic

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

Our thanks to the conference organisers for the invitation to participate and to colleagues who have collaborated with us in work on fossil lepidosaurs, notably: Magdalena Borsuk-Białynicka (Poland); Dan Chure, David Krause (USA); Makoto Manabe (Japan); Alan Tennyson (New Zealand); Yuan Wang (China); and Trevor Worthy (Australia). Jerry D. Harris (USA) and Sebastian Apesteguía (Argentina) commented on an earlier draft of the manuscript. Although her phylogenetic interpretations have been revised, Pamela Robinson played a major role in focusing attention on lepidosauromorph evolution, and on the enormous potential of fissure infills and other microvertebrate assemblages in uncovering the history of the group.

Fraser, NC (1994) Assemblages of small tetrapods from British Late Triassic fissure deposits. In: Fraser, NC, Sues, H-D (eds) In the shadow of the dinosaurs: early Mesozoic tetrapods. Cambridge University Press, Cambridge, MA.Google Scholar

Kaye, FT, Padian, K (1994) Microvertebrates from the Placerias Quarry: a window on the Late Triassic vertebrate diversity in the American South West. In: Fraser, NC, Sues, H-D (eds) In the Shadow of the Dinosaurs: early Mesozoic tetrapods. Cambridge University Press, Cambridge, MA.Google Scholar

Sues, H-D, Clark, JM, Jenkins, FA, Jr (1994) A review of the Early Jurassic tetrapods from the Glen Canyon Group of the American Southwest. In: Fraser, NC, Sues, H-D (eds) In the shadow of the dinosaurs: early Mesozoic tetrapods. Cambridge University Press, Cambridge, MA.Google Scholar